1. Field of the Invention
The present invention relates to a communication system for, for example, transmitting data by air, an apparatus and a method for controlling communications, an apparatus and a method for communications, and a computer program and more particularly to a communication system, an apparatus and a method for controlling communications, an apparatus and a method for communications, and a computer program wherein data transmission is made between mobile terminals through the mediation of base stations which provide cells wherein communication can be conducted.
In further details, the present invention relates to a communication system, an apparatus and a method for communications, an apparatus and a method for communications, and a computer program wherein the cells of a plurality of base stations are deployed in plane to implement wider communication ranges and more particularly to a communication system, an apparatus and a method for controlling communications, an apparatus and a method for communications, and a computer program wherein data transmission is made between mobile terminals without tree-structuring base stations or establishing base station control stations which control a plurality of base stations to manage information on routes in a centralized manner.
2. Description of Related Art
Mobile communication arises from the discovery of electromagnetic waves and has been researched and developed to meet the necessity for communication with ships, airplanes, and trains. The targets to which communication is conducted have been expanded to automobiles and humans. In addition to communications by telegraph and by telephone, computer data and multimedia contents including images have been transmittable.
Owing to the recent advances of manufacturing technology and the like, mobile terminals have been rapidly reduced in size and lowered in price. Further, because of the improvement of information and communication services and the like, mobile terminals have been personalized as seen in the example of cellular phones. Moreover, liberalization of the telecommunications sector, reduction in communication charges, and other factors have increasingly expanded the tiers of users.
Mobile communication is basically such that a mobile terminal, such as car telephone or cellular phone, finds the nearest base station and communicates radio waves between the mobile terminal and the base station. The communication range that a radio wave from one base station can reach is referred to as “cell.” A cell is usually a circle with a specified radius, whose center is located at a base station antenna. Base stations are installed at certain special intervals to deploy a plurality of cells provided by the base stations in plane, and a wide service area is thereby built.
The reason why mobile communication systems use cells is the advantages derived therefrom, including that the reach of radio waves from a base station is limited to within the cell thereof and thereby the same frequencies can be repeatedly used by other cells and the limited frequency resources can be effectively used, and that a service area is compartmentalized into cells and thereby the transmission power for communications is reduced for shrinking the size of and saving the power of mobile units which are usually packaged as battery-driven portable devices. With increase in the number of cellular phone users and the like, the size of cells has been increasingly reduced.
FIG. 1 schematically illustrates the configuration of base stations in an existing cellular radio communication system, such as PDC (Personal Digital Cellular) and GSM (Global System for Mobile Communication). Base stations are in general connected in tree structure, and, as shown in the figure, the communication system comprises base stations which are scattered here and there and manage their own cells; base station control stations which control a plurality of base stations in respective areas; mobile communication switching centers which implement switching between a plurality of base station control stations; and core networks which connect together remote mobile communication switching centers.
In such a tree-structured base station configuration, control of routing between base stations is facilitated because each station located at a branch point of the tree manages information on routes indicating how control data and user data destined for mobile terminals should be delivered.
More specifically, a base station in the undermost layer, in response to a request to send occurs in the cell, makes transfer in the cell if the target of the transmission is found in the same cell, and transfers the request to send to the base station control station at the immediately upper level if the target is not found. Similarly, the base station control station completes the request to send in its own range of control if the target of the transmission is found in the range of control, and transfers the request to send to the mobile communication switching center at the next upper level if the target of the transmission is not found. The mobile communication switching center transfers the request to send to another mobile communication switching center through a core network if the target of the transmission is not found in the coverage of the switching center. Therefore, transmitted data from a mobile terminal is delivered to the destination through base station control stations which control a plurality of base stations, or through mobile communication switching centers.
Consideration will be given to a case where transmission is made from a mobile terminal 1-1 to another mobile terminal 1-5, for example. A base station control station 1-6 holds information indicating that the mobile terminal 1-5 is camping in the cell of a base station 1-4, and as a result, data from a base station 1-2 is delivered to the mobile terminal 1-5 as the target of the transmission through the base station control station 1-6 and the base station 1-4.
The above-mentioned tree-structured configuration of base stations is suitable for large-scale radio communication systems, such as cellular radio communication systems, but not for small-scale radio communication systems comprising cells called micro-cells and pico-cells.
One of the reasons for the unsuitability is that base stations need be connected with one another using wires and installation work for wired connection line (approach line) and the maintenance and management thereof impose heavy burden on communication common carriers. In the micro-cell or pico-cell environment, the condition of propagation of radio waves from base stations largely fluctuates due to surrounding buildings and the like, and the arrangement and configuration of base stations need be changed on a case-by-case basis to cope with this. Another reason for the unsuitability is that the above-mentioned tree-structured configuration cannot flexibly cope with this change.
High load is applied to control stations located at branch points of the tree, and, to add a new base station, the information on the base station need be stored in the control station in advance. Thus, it is difficult to easily install a new base station.